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Report: Groundwater-quality Monitoring—October 25,2002: 7500 West Eleventh Street, Tracy, CA. Page 10 Comparing results of analyses of samples of groundwater recovered on October 25, 2002 with those from July 29, 2002, it is noted that the concentrations of diesel-range hydrocarbons in the groundwater sample recovered from monitoring well MW-3 fell from 490 gg/L to 210 gg/L and the gasoline-range hydrocarbons fell from 4,800 gg/L to 34 gg/L, with commensurate large decreases in the concentrations of the BTEX compounds and MTBE. MW-3 is a short distance down gradient from the former location ' of the fueling station's pump islands where the preponderance of the leakage of fuels to the subsurface had occurred. Similar decreases were seen in the results of the analyses of the sample recovered from Monitoring Well MW-4. In that sample no detectable concentration of TPH(d), TPH(g) or BTEX were detectable and by October 25, 2002, the concentration of MTBE had fallen to 510 µg/L, compared to the 860 gg/L present in the sample recovered on July 29, 2002. The decreases in concentrations of analytes of concern in groundwater noted above can be attributed to the falling water table. As the water table falls, a zone of soil that is affected by analytes of concern that is present at the elevation of the previously higher water table elevation is no longer in contact with the groundwater; thus, desorbtion of components of fuel hydrocarbons from that mass ceases. Similarly, there was 100 gg/L of diesel and 189 gg/L of gasoline in the October 25 sample from Monitoring Well 12 (see Figure 2 for location) compared to 1,500 gg/L and 1,700 gg/L, respectively, of those analytes in the sample recovered from that well on July 29, 2002. r - Inspection of Table 2 also shows both increases and decreases in the concentrations of analytes of concern in the groundwater that can be correlated with water tables elevations shown in Table 1. However, the trend of.concentration over time is downward in samples recovered in the area along the West Eleventh Street frontage of the 7500 West Eleventh Street property, where fuel hydrocarbons were released into the subsurface. -7 was first analyzed in May 2000, it was affected by When groundwater in Well MW relatively moderate concentrations diesel- and gasoline-range petroleum hydrocarbons (at concentrations of 1,300 gg/L and 1,400 gg/L, respectively) and MTBE, at 22 gg/L. However, over the next two years, the concentrations of both diesel and gasoline in samples recovered from that well exhibited a significant upward trend. As the concentration of those analytes increased, MTBE was no longer detected. That is almost ' certainly due to its presence being obscured in the chromatographs rather by its actual absence. By April 11, 2002, it was evident that there was floating product in Well MW-7 E and it was measured to have an apparent thickness of 0.3 ft. On July 29, 2002 the t apparent thickness of floating product had reduced to 0.18 ft., but when it was measured on October 25, 2002, it had again increased in thickness to 0.48 ft. ' i ` An increase in the apparent thickness of floating product in a well that is perforated both above and below the maximum elevation reached by groundwater is commonly seen in { association with a falling water table. Conversely, as was the case in the period from sic